Cap for self-contained biological indicators

ABSTRACT

A biological indicator is disclosed for determining the efficacy of a sterilization cycle using hydrogen peroxide vapor at subatmospheric conditions. The biological indicator includes a vial formed of a substance which resists retention of residual sterilant while also maintaining vial clarity. Also disclosed is an improved cap design which facilitates activation of the biological indicator after the sterilization cycle is complete.

This is a divisional of application Ser. No. 08/125,773 filed on Sep.24, 1993, now U.S. Pat. No. 5,405,580.

BACKGROUND OF THE INVENTION

In the field of sterilization of articles, it is desirable to insure thequality and integrity of a sterilization environment, and to ensure thata particular load of articles to be sterilized has in fact been exposedto an environment which would have adequately killed bacterialmicroorganisms. To this end, it is known in the art to provide a"biological indicator", a compact device which insures the efficacy of asterilization cycle.

Several types of biological indicators are known in the art. Suchsystems are taught, for example, in U.S. Pat. Nos. 4,304,869 and4,461,837. These systems offer a self-contained unit which permits abiological sample to be exposed to a sterilizing environment (along withthe desired articles to be sterilized), with the unit simultaneouslysealing and immersing the biological sample in a growth-inducing mediumupon activation of the unit. The growth-inducing medium is mixed with adye which vividly changes color to indicate spore growth. A color changeindicates the presence of spores which suggests that the batch had notbeen properly sterilized.

In the prior art, the vial is fashioned from polycarbonate, a materialwhich works well in the conventional steam and ethylene oxidesterilization processes. With the advent of hydrogen peroxide vaporsystems, it was discovered that the polycarbonate vials did not workwell with this sterilant.

In order for typical self-contained biological indicators to beeffective, it must be possible to determine the color changes in thegrowth-inducing medium. In typical devices, the fluid changes color frompurple to yellow to indicate contamination.

Hydrogen peroxide sterilization also creates other difficulties. Thematerial used in the spore carrier of the prior art was found to also beincompatible with hydrogen peroxide. Failure of the spore carriermaterial during a hydrogen peroxide sterilization cycle would yielderroneous results or even total failure of the biological indicator.

In addition, in the prior art, the interior surface of each prong of thecap typically is concave and designed to form around the surface of theampule. In this configuration, as seen in FIG. 6A, there issubstantially complete contact between the prong and ampule and theforce of fracture transmitted by the prongs during sealing andactivation is spread out over a large area. This structure wasdiscolored to be ineffective with the present invention.

SUMMARY OF THE INVENTION

The present invention overcomes the problems inherent in usingconventional biological indicators with hydrogen peroxide based systems.A significant number of inferior outgrowth results were found to be dueto a raw material incompatibility which resulted in significant residuallevels of hydrogen peroxide adhering to the polycarbonate vial. Suchresidual peroxide could injure or continue to kill remaining spores orinhibit their outgrowth in the biological indicator after thepredetermined sterilization cycle has been completed and after theproduct has been removed from the sterilizer. According to the presentinvention, there is provided a biological indicator havingmicroorganisms and a growth medium contained in a capped vialconstructed of clear polypropylene. The inventors have discovered thatclear polypropylene provides particularly good performance and retains alow level of residual hydrogen peroxide as compared to other materialsavailable for use as a vial. In combination with this good performance,the present vial material is also substantially transparent, and thusoffers good "clarity", permitting more precise determination of thecolor change pointing to non-sterile conditions.

The present invention also provides a spore carrier made of ultrafineglass fibers in an acrylic binder which has been found to be inert toattack from hydrogen peroxide and the chosen bacteriological spores.Thus, the spore carrier according to the present invention will not failduring use.

The prongs of the present invention are configured to provide linecontact with the ampule. This structural feature has been found mostadvantageous for ampule activation, as used with a polypropylene vial.

The foregoing operation and configuration, along with the other featuresand advantages of the present invention may be realized and obtained bythe following detailed description, the accompanying drawings and theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general view showing the biological indicator;

FIG. 2 is an exploded assembly view showing the components of thebiological indicator;

FIG. 3 is a sectional view illustrating the manner with which sterilantis admitted into the biological indicator;

FIGS. 4A and 4B show the activation of the biological indicator afterthe sterilization cycle;

FIGS. 5A and 5B are oblique views comparing the caps of the prior artand the preferred embodiment showing differences in the prongs; and

FIGS. 6A and 6B are cross-sectional views comparing the performance ofthe caps of the prior art and the preferred embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIGS. 1 and 2, the preferred embodiment of the presentbiological indicator 10 essentially includes a cap 11 with threeflexibly projecting "prongs" 12 which are configured to substantiallysurround and contact an ampule 13 of growth-inducing medium. The cap 11and ampule 13 are then fitted inside a vial 14 into which is alsoinserted a spore carrier 15. During activation of the device, the prongs12 are forced against the ampule 13, thus fracturing it and releasingthe growth-inducing medium.

The spore carrier substrate typically is inoculated with anapproximately 10⁴ population of spores of a known strain of Bacillusstearothermophilus (B.st.). The combination of the spore sample and thevial/cap assembly medium mimics the resistance of a 1×10⁶ population ofB.st. on a stainless steel carrier when subjected to hydrogen peroxidevapor sterilant at less than atmospheric pressure.

The biological indicator 10 is designed to admit hydrogen peroxidesterilant vapor onto the spore carrier 15 as shown in regard to FIG. 3.In the inactivated state, the cap 11 is configured on the vial 14 insuch a way as to permit vapor/gas between the interior of the vial andthe outside environment.

During sterilization, hydrogen peroxide vapor can enter the vial chamberby way of a "tortuous path" 16 whereby the vapor must negotiate at leasttwo 90° bends in order to enter the vial. The sterilant entering thechamber then acts upon the spore carrier 15 in order to expose thecarrier to the same environment encountered by the other articles to besterilized.

Upon completion of the sterilization cycle, the biological indicator 10is removed from the sterilizer and thereby is exposed to a non-sterileenvironment which may contain airborne microorganisms. Because therewill be little or no pressure differential between the exterior and theinterior of the container 10, there is minimal or extremely lowprobability that the non-sterile environment, e.g. air, will flow intothe chamber of the vial 14, carrying with it airborne microorganisms.Access to the chamber of the vial 14 by airborne microorganisms may begained only if the microorganisms negotiate the tortuous path 16. Suchmigration will be unlikely to occur because the microorganisms areincapable of independent movement and must be carried along by anynegligible air currents that would traverse the tortuous path.

After being sterilized with the desired articles in the manner describedabove, the biological indicator is activated as shown in FIGS. 4A and4B. The top of the cap 11 is pressed down, sealing the top of the vial14, cutting off further vapour transmission. As the cap 11 is presseddown, the prongs 12 are pushed into the taper of the vial 14, whichforcibly crush the ampule 13, releasing the growth-inducing medium uponthe spore carrier 15. Should any viable spores remain (i.e. should thesterilization cycle be inadequate), they will thrive on thegrowth-inducing medium in the sealed vial 14. Should spore growth occur,the dye in the growth-inducing medium will vividly change color inresponse to well known biochemical reactions produced by B.st.

The present invention addresses the need to ensure accurate test resultswhile at the same time offering sufficient "clarity" and abrasionresistance, including resistance to nicks, scratches and peeling.Polycarbonate was found to contain a high level of residual hydrogenperoxide which affects the accuracy of a sterilization test. In seekingto replace polycarbonate, several different vial materials were firstselected for a desired level of clarity, including polymers from thegroup acrylic, polypropylene and modified styrene such asstyrene-butadiene copolymer, sold under the trade name "KRO3" byPhillips 66 Company. These vials were subjected to a Minimum InhibitoryConcentration (MIC) test to determine a concentration of residualhydrogen peroxide which would permit an acceptable outgrowth of B.st.after a desired interval.

The steps of the MIC test are as follows: vials were formed from theselected materials using the standard techniques of injection molding. Astudy sample of these vials was then run through a hydrogen peroxidesterilization cycle. At the end of this cycle, a 3 milliliter samplecontaining a standard quantity of B.st. spores was injected into eachvial. The vials were then maintained at an incubation temperature of55°-60° C. At the end of 48 hours, the outgrowth of B.st. spores wasobserved. It was concluded that spore samples having residual hydrogenperoxide in solution at an initial concentration of less than 40 μg/mlwould grow out to the desired level within 48 hours. Samples havinghigher levels would display inhibited outgrowth and would not grow outas quickly, thus offering inconclusive sterility results. Polycarbonatehad a higher level and did not grow out for 72 hours due to the levelsof residual hydrogen peroxide. After processing through an extendedcycle, initial levels of hydrogen peroxide in solution with the sporesample were measured by subjecting the sample to a variation of theXylenol Orange Spectrophotometric test for measuring low concentrationsof hydrogen peroxide, such techniques being known in the art.

Upon completing this MIC test, it was observed that the modified styrenematerials had residual hydrogen peroxide concentrations under theacceptable level of 40 μg/ml, and thus displayed desirable outgrowthafter 48 hours. However, these materials scratched easily and thus arenot the preferred embodiment. Acrylic had an acceptable abrasionresistance, but had a high level of residuals.

It was found that clear polypropylene (in the preferred embodiment,Exxon Escorene 1105) was found to offer significantly better resultsthan the prior art polycarbonate. This variety of polypropylene has beenfound to successfully meet all three criteria: clarity, abrasionresistance and low residual level.

The reason that polypropylene performs unusually well for thisapplication is not fully understood. Presumably it is a function of themolecular structure of polypropylene. In the preexisting prior artbiological indicators, the caps 11 had been made of opaque polypropyleneand so the desirable resistance to hydrogen peroxide is realized withoutreplacing the existing cap material.

On a molecular level, polypropylene has a very linear polymer chain. Thesidechains of this polymer have a very low molecular weight, especiallyas compared to polycarbonate which has large sidechains of relativelyhigh molecular weight. These sidechains are believed to promote theretention of hydrogen peroxide, and thus produce the high levels ofresiduals that have been observed.

In addition to its providing resistance to the hydrogen peroxidesterilant, the Exxon Escorene 1105 clear polypropylene also was chosenfor being less opaque, thus improving the accuracy of test results.Ordinary polypropylene suffers from being somewhat "cloudy." Escorene1105 contains additives that improve its clarity. Using this type ofclear polypropylene, clarity is significantly improved and consequently,accurate monitoring of hydrogen peroxide sterilization cycles isfacilitated, with the result that "false positive" test results can bereduced considerably. Polypropylene also works well in injection moldingequipment, being of a sufficiently low viscosity that it is easilymolded.

While the discovery of the hydrogen peroxide resistance of polypropyleneis most applicable in the present solution to the problem withbiological indicator vials, it is contemplated that this material'sproperties may be exploited in other applications adapted for use inhydrogen peroxide systems. For example, worker safety is always aconcern and hydrogen peroxide is a well-known corrosive material. It maybe useful to form the components of a variety of implements frompolypropylene in order to protect workers from exposure risk. Suchimplements include personnel monitoring systems as are known in the art.

Another application is the use of polypropylene elements in a"biological challenge test", a kit which includes the general elementsof a biological indicator, and which simulates a "worst case"contaminated load, thus insuring the overall efficacy of a sterilizationcycle. Polypropylene also makes a suitable packaging "barrier film",used as a cover layer for the articles inserted into a "heat seal" pouchwhich is placed in a sterilizer unit as known in the art.

Referring now to FIG. 5B, the design of the prongs 12 is also part ofthe present invention. Polypropylene has been found to be problematicusing prior art structures because it is somewhat less rigid than theprior art polycarbonate. Even Exxon Escorene 1105, with its improvedmechanical strength, does not provide rigidity equal to polycarbonate.The vials of the present invention tend to deform upon application ofthe activation force while using caps with the conventional prongs. Thisdeformation necessitates a very large application of force upon the cap11 in order to fracture the ampule 13.

As shown in FIG. 6B, this problem is overcome in the present inventionby configuring the interior surfaces 17 of the prongs 12 to contact theampule 13 over a minimal area. In the preferred embodiment, these areconvex in shape. Upon activation, greater pressure is exerted upon theampule 13 as the force is more concentrated onto a smaller area. As aresult, the ampule fractures more easily and at a smaller application offorce, thus facilitating activation without deforming the polypropylenevials.

The spore carrier 15, according to the present invention, is formed froma composite of an ultrafine glass fiber material in an acrylic binder,of the type used in solution filtration systems. It has been discoveredthat this composite offers high chemical resistance to the hydrogenperoxide sterilant and provides a stable substrate for the B.st. spores.Consequently, failures of the spore carrier 15 are greatly diminished.

The foregoing description of the preferred embodiment has been presentedfor purposes of illustration and description. It is not intended to belimiting insofar as to exclude other modifications and variations suchas would occur to those skilled in the art. Any modifications orvariations such as would occur to those skilled in the art in view ofthe above teaching are contemplated as being within the scope of theinvention.

What is claimed is:
 1. A cap used with a biological indicating systemcomprising:a plurality of flexible projections depending from said capand constructed to wedge between an ampule and a vial and therebyfracture said ampule as said cap is moved from a first position to asecond position; a convex surface upon the interior of each of saidflexible projections, configured such that the surface substantiallymakes line contact with said ampule, and further configured so that whensaid cap is moved from said first position to said second position, theforce is placed upon the line of contact with said ampule, facilitatingthe fracture thereof.